The Blackcap will be familiar to many as a summer visitor, heard singing from scrubby cover or chattering its ‘two pebbles brought together’ alarm call at those who stray close to a nest with well-developed chicks. For others, however, the species is increasingly seen as a winter visitor to garden feeding stations, where it looks oddly out of place alongside the finches and tits.
These two views underline a changing pattern to the occurrence of this short-distance migrant in Britain and Ireland; Blackcaps from central Europe have been wintering in urban areas of Britain with increasing frequency over the past 60 years, rather than migrating south-west to Spain. Research has revealed that this new migration strategy is genetically encoded, being maintained on the breeding grounds through reproductive isolation (individuals wintering in Britain mate with other birds that do the same thing) and fitness benefits (birds wintering in Britain fledge more chicks).
Why we are seeing more Blackcaps wintering in Britain?
Comparison of winter distribution maps from BTO-led Atlas projects reveals the increase in our wintering population and an expansion in the bird’s winter range. The core distribution, however, is centred on the south and west of Britain, which happen to be the regions where the winter weather is more favourable. Examination of BTO Garden BirdWatch data show a similar pattern. Winter conditions in Britain have become milder over time because of a changing climate and this may have led to improved overwinter survival rates for the Blackcaps choosing to winter here and supporting the observed increase.
Early observations of wintering Blackcaps in British gardens coincided with the wider introduction of commercial wild bird foods, so it is conceivable that provision of these has also had some influence on the evolution of the new migration strategy. Analysis of the BTO’s Garden BirdWatch data by Kate Plummer and others, has unravelled the underlying mechanisms driving the evolution of this change in migration route. The study found strong evidence that Blackcap occupancy rates of gardens in winter are influenced by both supplementary food and temperature. Blackcaps were recorded more often at sites that provisioned food more frequently and showed a preference for those that had a warmer local climate. Perhaps most interestingly, Blackcap occurrence has become more strongly associated with supplementary feeding over time.
What BTO evidence shows
This work provides the first direct evidence of the underlying mechanisms that have influenced the contemporary evolution of migratory behaviour in Blackcap. Over a 12-year period, Blackcaps have become increasingly associated with the provision of supplementary foods in British gardens and the reliability of that provisioning is influencing their distribution at the national scale. The findings suggest that an improving winter climate is also likely to have enabled Blackcaps to expand their wintering range into Britain.
The increasing association with supplementary food over time suggests that Blackcaps are adapting their feeding habits to exploit human-provisioned foods, complementing recent evidence that those Blackcaps migrating to Britain in winter are diverging phenotypically, as well as genetically, from those that winter in Spain. Blackcaps wintering in Britain have relatively narrower and longer beaks than those wintering in Spain, suggesting that British migrants have adapted to a more generalist diet. We know from Garden BirdWatch observations that the birds have a preference for fats and sunflower hearts.
What does the future hold?
A changing climate, with an associated improvement of wintering conditions further north, may see the development of sedentary behaviour in some migrant populations – think of the increasing numbers of Chiffchaffs wintering in Britain rather than migrating south. This raises the question of whether any of the Blackcaps that make up our breeding population – breeding here but currently wintering around the western end of the Mediterranean – might start wintering here, rather than heading south. To date, there have only been six recorded instances of British breeding Blackcaps remaining in Britain during winter. There is also the possibility that the central European Blackcaps visiting us might increasingly remain on their central European breeding grounds, though this would be dependent on local weather conditions and the availability of supplementary food. Currently, the pattern of food provision in German gardens is rather different from what we see here in Britain.
The BTO study provides new and timely evidence of the role that human activities can play in shaping the evolutionary trajectories of wild bird populations and the response of the Blackcap to these opportunities may be important longer term. Changing environmental conditions may threaten the viability of traditional wintering areas in southern Spain, making the new opportunities further north all the more important.
The value of citizen science
BTO research into Blackcap wintering behaviour would not have been possible without the generous support of BTO Garden BirdWatchers and the weekly records that they collect and submit. The large scale of the Garden BirdWatch project – observations from nearly 4,000 gardens were used in this particular study – provides the level of information essential for these types of analyses and underlines the role that citizen science can play in improving our understanding of birds and the processes that shape their populations.